1. Field of the Invention
The present invention relates to an elastic convolver utilizing the nonlinearity of surface acoustic waves, and more particularly, to an elastic convolver utilizing a structure in which a ZnO piezoelectric thin film is deposite on a piezoelectric substrate composed of an LiNbO.sub.3 piezoelectric single crystal.
2. Description of the Prior Art
An elastic convolver is one type of signal processing device utilizing the nonlinear behavior of a piezoelectric body, which is an operating device for performing convolution integration of two input signals. As this elastic convolver, a structure in which a multistrip coupler (hereinafter referred to as an MSC) is incorporated has been conventionally known. One example of this known elastic convolver is shown in FIG. 8.
The elastic convolver 1 is constructed using as a piezoelectric body a rectangular piezoelectric substrate 2 composed of an LiNbO.sub.3 piezoelectric single crystal. Input interdigital transducers (hereinafter referred to as input IDTs) 3 are respectively formed in the vicinities of both end surfaces 2a and 2b of the piezoelectric substrate 2. The input IDTs 3 are constituted by a pair of comb electrodes each having electrode fingers which are inserted into each other.
Furthermore, a waveguide path 4 extending parallel to the direction of surface wave propagation is formed in the center of an area between the input IDTs 3. MSCs 5 are respectively formed between the waveguide path 4 and the input IDTs 3.
In the elastic convolver 1, if input signals are applied to the input IDTs 3, surface acoustic waves which are excited by the input signals are respectively propagated in directions indicated by arrows A and B. The surface acoustic waves are respectively compressed in the MSCs 5 and then, are overlapped with each other in the waveguide path 4, so that an output signal is taken out.
The performance of an elastic convolver is generally indicated by an efficiency F and a BT product (B represents a bandwidth and T represents integration or process time). It has been desired to improve the efficiency F and the BT product.
Since the elastic convolver utilizes surface acoustic waves as described above, it is considered that a piezoelectric substrate 2 having a large electromechanical coupling factor and having significant nonlinearity may be used in order to increase the efficiency F.
On the other hand, it is reported that when IDTs are formed on a piezoelectric substrate composed of an LiNbO.sub.3 piezoelectric single crystal to excite surface acoustic waves, a ZnO piezoelectric thin film is deposited on the surface of the LiNbO.sub.3 piezoelectric substrate, thereby to obtain a larger electromechanical coupling factor (an article by A. Armstrong. et al., Proc. 1972 IEEE Ultrason. Symp. pp. 370 to 372 (1972), and an article by Nakamura et al., Proceedings of Japanese Conference on Acoustics, October 1991, pp. 953 to 954).
Therefore, a ZnO piezoelectric thin film is formed so as to cover the input IDTs 3, the MSCs 5 and the waveguide path 4 on both surfaces of the piezoelectric substrate 2 in the elastic convolver 1 shown in FIG. 8 to manufacture an elastic convolver. Consequently, it is confirmed that the manufactured elastic convolver is increased in efficiency, as compared with the conventional elastic convolver 1.
However, it is confirmed that the bandwidth and particularly, the bandwidth (3 dB attenuation bandwidth) at which the amount of attenuation is decreased by 3 dB is significantly decreased, although the efficiency is increased. That is, the elastic convolver can process a signal having a wider spectrum when the bandwidth is larger. Accordingly, the large bandwidth is required. If the ZnO piezoelectric thin film is formed on the entire surface as described above, however, it is found that the bandwidth is decreased, although the efficiency is increased, which is unfavorable.